Flat panel field emission displays (FEDs), like standard cathode ray tube (CRT) displays, generate light by impinging high energy electrons on a picture element (pixel) of a phosphor screen. The excited phosphor then converts the electron energy into visible light. However, unlike conventional CRT displays which use a single or in some cases three electron beams to scan across the phosphor screen in a raster pattern, FEDs use stationary electron beams for each color element of each pixel. This allows the distance from the electron source to the screen to be very small compared to the distance required for the scanning electron beams of the conventional CRTs. In addition, FEDs consume far less power than CRTs. These factors make FEDs ideal for portable electronic products such as laptop computers, pagers, cell phones, pocket-TVs, personal digital assistants, and portable electronic games.
One problem associated with the FEDs is that the FED vacuum tubes may contain minute amounts of contaminants which can become attached to the surfaces of the electron-emissive elements, faceplates, gate electrodes, focus electrodes, (including dielectric layer and metal layer) and spacer walls. These contaminants may be knocked off when bombarded by electrons of sufficient energy. Thus, when an FED is switched on or switched off, there is a high probability that these contaminants may form small zones of high pressure within the FED vacuum tube.
Within an FED, electrons may also hit spacer walls and focus electrodes, causing non-uniform emitter degradation. Problems occur when electrons hit any surface except the anode, as these other surfaces are likely to be contaminated and out gas.
The problems associated with contaminants, electron bombardment and out gassing can lead to brightness variations from row-to-row in an FED device. These brightness variations can be most pronounced around the rows that are nearby spacer walls. Spacer walls are placed between the anode and emitters of an FED device and help maintain structural integrity under the vacuum pressure of the tube. One cause of brightness variations of rows nearby spacer walls results from a non-uniform amount of contaminants falling onto the emitters that are located near spacer walls. More contaminants falling on these emitters makes rows dimmer or brighter that are located nearby the spacer walls.
Another factor leading to brightness variations row-to-row is that electrons may strike the spacer walls thereby causing ions to be released which migrate to the emitters. These ions may make the rows closer to the spacer walls actually get brighter. Also, over the life of the tube, gasses exit the faceplate and the existence of the spacer walls causes a reduced amount of these gasses to be absorbed by the emitters near the spacer walls compared to those emitters that are located farther away from the spacer walls. As a result, the cathodes of the emitters located near the spacer walls are left in relatively good condition thereby leading to brighter rows near the spacer walls.
Unfortunately, the human eye is very sensitive to brightness variations of rows that are close together. These variations can cause visible artifacts in the display screen that degrade image quality.
It would be advantageous to reduce or eliminate brightness variations of the rows of an FED device. More specifically, it would be advantageous to reduce or eliminate brightness variations for rows located nearby spacer walls.